Fuel oils can be divided into two primary classifications, i.e., distillate fuel oils, and residual fuel oils. Distillate oils are composed entirely of material vaporized in refinery distillation towers; consequently, they are clean and free of sediments, relatively low in viscosity, and they contain no inorganic ash.
Residual fuel oils, however, contain fractions that cannot readily be vaporized by heating. These fractions are black and viscous, and they include any organic ash components originally present in the crude. In some cases, the whole crude itself is used as a residual fuel.
Distillate fuel oils, being low in sulfur, free of ash, and relatively easy to handle, are used in applications where such qualities are more important than fuel prices, for example, in home heating. However, where cost is an important consideration, and large volumes of fuel are required, for instance, in electric power generation, in producing industrial steam, for process heating and the like, residual oils are usually employed as the fuels of choice.
As indicated, not all of the residual oil can be vaporized even after vigorous heating; consequently a relatively large amount of unburned residue is accumulated during the burning process. Such material collects wherever the burning takes place, for example, at the bottom of boilers, in kilns, etc., causing problems including corrosion and other unwanted chemical reactions. Furthermore, in the case of direct-fired equipment, such accumulations interfere with heat transfer, leading to wasted energy, as well as causing extended down-time as a result of the need to more frequently take the equipment off-line for cleaning and turn-around. In addition to the costs entailed in reduced operating time, such maintenance is hard on the equipment and frequently causes added wear and tear thereto.
Even before the residual oil is burned, however, deposits therefrom tend to accumulate in fuel lines and in fuel storage tanks, causing cleaning problems. In any event, the fuel value represented by the inability to conveniently burn such deposits amounts to a significant energy loss over time.
Beyond the preceding disadvantages, however, and due to the incomplete combustion of the residual oil components, the burning process results in the formation and release to the environment of undesirable amounts of pollutants such as smoke, carbon monoxide and sulfur compounds, all of which have a detrimental affect on the atmosphere, and such release in many instances is contrary to law.